There is a requirement for an increase in an output power of a base station with an increase in an amount of communication data in wireless communication. As a device for increasing the output power of the base station, there is a High Power Amplifier (HPA). The HPA is required not only for the increase in the output power but also for a high power efficiency. Recently, therefore, in order to amplify the output power and to improve the power efficiency, a high power amplifier has been introduced as a Composite-High Power Amplifier (C-HPA) that includes a plurality of HPAs.
FIG. 6 is a block diagram of LINC (LInear amplification with Nonlinear Components) based on a related example. As illustrated in FIG. 6, LINC 200 includes two amplifiers 241 and 242. In the LINC 200, an input signal is separated by a signal component separator 210, and the separated signals pass through D/A (Digital to Analog) converters 221 and 222, quadrature modulators 231 and 232, and the amplifiers 241 and 242, respectively, and are then combined by an adder 250. During the process, different transport delays τ1 and τ2 occur in the arms along which the two separated signals reach the two amplifiers 241 and 242, respectively. An output signal y of the LINC 200 represents the vector sum of output signals y1 and y2 of the amplifiers 241 and 242, respectively. Therefore, in order that the LINC 200 generates the output signal y without any distortions due to combining, it is desirable that the imbalance for amplitudes, phases, and delays between the two arms are compensated.
Of imbalances for amplitudes, phases, and delays, the imbalance for amplitudes and phases may be compensated by the LINC. FIG. 7 is a diagram of a process in which the LINC based on the related example compensates the imbalance for amplitudes and phases. As illustrated in FIG. 7, LINC 300 has substantially the same configuration as that of the LINC 200 illustrated in FIG. 6, except for compensators 361 and 362 provided in the arms, respectively. Therefore, the detailed description of the LINC 300 is omitted. The compensators 361 and 362 multiply signals input from signal component separators 310 and 390 by predetermined different complex coefficients K1 and K2, respectively, to compensate the imbalance for amplitudes and phases between the arms. However, it is difficult for the LINC 300 to compensate the delays in the arms.
A DPD (Digital PreDistorter) that includes a DLL (Delay Looked Loop) circuit is a technology for compensating delays occurring in the amplifiers, however, the DLL is a technology for compensating individual delays occurring in the arms. Therefore, delay discrepancy between the amplifiers is difficult to be adjusted. In the case of the C-HPA such as LINC, if amplified signals are combined before the delay discrepancy is adjusted, an output spectrum degrades at out-of-band frequencies. In other words, at the out-of-band frequencies, in order to reduce interference with adjacent channels or to improve the use efficiency of the frequencies, it is preferable to decrease out-of-band spectrum. However, when the delay discrepancy has occurred between the amplifiers, the level of the out-of-band spectrum significantly increases at such a value that the out-of-band frequency exceeds 10 MHz.